RENAL CLEARANCE OF SODIUM IN THE DOG: EFFECT OF INCREASING SODIUM LOAD ON REABSORPTIVE MECHANISM

Abstract

When Na load to the tubular reabsorptive mechanism is increased by a combination of increase in glomerular filtration rate and increased plasma Na concn. resulting from rapid infusion of 5% NaCl soln., the total amt. reabsorbed tends to reach a limiting value, even as load is further increased. This is accompanied by marked increase in urinary excretion of Na. In some animals the limitation of reabsorption is abrupt; in others it is more gradual, so that different contours of the curves relating reabsorption to load are displayed. These differences in contour are attributable to changes in filtration rate accompanying the loading. The conclusion is reached from the data in which reabsorption is expressed in terms of 100 ml. of glomerular filtrate that the kinetics of Na transfer, presumably hormonally-regulated, are not immediately altered by sudden changes in Na plasma concn., and continue to reabsorb a constant amt. so that any excess imposed on the mechanism is immediately excreted. Alterations in the ability of the tubular mechanism to reabsorb Na were seen late in the expts. Decreased reabsorption was apparent,presumably caused by modification of hormone concn. of the blood resulting from the effects of hypertonic salt.